1. Field of the Invention
This invention relates to a light scanning device for conducting deflection control of the light beam of a laser beam in accordance with an image signal in a electrophotographic type of recording device or the like in which a print operation is carried out at a high speed in accordance with an image signal transmitted from a computer, and more particularly to a bearing device of the light scanning device.
2. Description of Related Art
An electrophotographic type of recording device, for example, a laser beam printer, has been recently widely used as a recording device for recording image information from a computer. In general, the recording device includes a light scanning device, such as disclosed in U.S. Pat. Nos. 5,151,586 and 5,274,398, for conducting a deflection control of a light beam in accordance with image information to sweep the light beam on a photosensitive medium, a charging device for charging the photosensitive medium, a developing device for developing a latent image which is formed on the photosensitive medium by the light beam, and a transfer and fixing device for transferring and fixing the image developed by the developing device onto a recording medium, such as a sheet.
The light scanning device used in the electrophotographic type of recording device includes a reflection mirror, a bearing for rotatably supporting a rotational shaft of the reflection mirror, and a motor which is connected to the rotational shaft of the reflection mirror to rotationally drive the rotational shaft. The motor rotates the reflection mirror at about several thousands to ten thousand rpm to thereby carry out the deflection scanning of the light beam incident to the mirror surface of the reflection mirror.
It has been generally known that a rotor has two types of resonance modes which are called a cylindrical mode and a conical mode in which the rotational shaft of the rotor is whirled. The resonance rotating numbers (resonance frequencies) of both of the resonance modes as described above are determined by the mass of the rotor and the rigidity (stiffness) of a bearing for supporting the rotational shaft. Both of the resonance rotating numbers are increased as the mass of the rotor is decreased or the stiffness of the bearing is increased.
In the conventional light scanning device, the rotating number of the reflection mirror serving as the rotor is not very high and the following method has been adopted. That is, by supporting the rotational shaft of the reflection mirror through a large-size rigid bearing having high stiffness, the resonance rotating numbers of both of the resonance modes are increased, and by setting the operational rotating number (stationary rotating number) to be sufficiently lower than the resonance rotating numbers of both of the resonance modes, the whirl of the rotational shaft of the reflection mirror can be forcedly suppressed without considering the effect on the resonance modes.
However, in the light scanning device used in the conventional electrophotographic type of recording device, if adjustment of the dynamic balance of the reflection mirror or the positioning between the rotational shaft of the reflection mirror and the rotational center axis of the bearing is not performed with sufficiently high precision, the whirl of the rotational shaft of the reflection mirror occurs due to failure of the dynamic balance or the deviation (off-line) of the rotational center axis, resulting in a problem that the imaging position of the light beam which is scanned on the photosensitive medium is positionally deviated.
Further, the output speed of the recording device as described above must now be additionally increased and, thus, the rotational speed of the reflection mirror is required to be further increased to about 16,000 rpm. Accordingly, in the conventional light scanning device, when the rotating number of the rotor approaches the resonance rotating number, vibration amplitude of the rotational shaft is intensified due to the resonance phenomenon, so that degradation in performance, noise and a shortening of the lifetime occur. In some cases the device suffers instantaneous destruction.
In order to avoid the above problems, the conventional light scanning device is so designed that the resonance rotating number is sufficiently higher than the operational rotating number. Specifically, large pressurization is given for the ball bearing and a small gap is provided between a shaft portion and the bearing for a fluid bearing, that is, in both cases, the resonance rotating number is increased by increasing the bearing stiffness. Therefore, such a method induces a problem that the bearing resistance is increased when the pressurization of the bearing is increased and, thus, a large output is required for a driving circuit. A resulting problem is that the lifetime of the bearing is necessarily shortened. Further, the design of the small gap for the fluid bearing requires extremely high precision mechanical processing, thereby increasing the cost of the device.